The administration of 131I should strictly be avoided in pregnancy. Iodide concentrating capacity can be detected in the thyroid of the 1011 week fetus. If administration is within the first 8 weeks, the major problem is fetal whole body dose due to gamma emission from 131I in the maternal bladder (about 50100 μGy/ MBq of administered activity). This dose is reduced if the mother is well hydrated and voids frequently. Generally the lifetime risk of fatal cancer is considered to be 1015% per Sv (stochastic).
If the fetus is more than 8 weeks post conception, the developing fetal thyroid may accumulate iodine. Fetal thyroid doses are very high which can cause significant fetal thyroid damage leading to permanent hypothyroidism. The whole body fetal dose is likely to be less than 100 mSv and would not necessarily indicate termination of the pregnancy on the basis of the exposure alone. The mother should, however, be given usual doses of replacement thyroid hormone. If discovered within 12 hours of administration, 60130 mg of stable KI is recommended. Since 131I is concentrated in maternal milk, breast feeding is contra-indicated after radioiodine administration to the mother. A radiation dose of approx 550 cGy would be delivered after the administration of 1mCi to the mother.
This presentation will consider the risk assessment after exposure to 131I with case studies taken from clinical practice. Dose-dependent effects of 131I on gonadal function will also be discussed.